Yuya Ishii has pitched for several teams in Nippon Pro Baseball. He is basically deaf in his left ear.

Ishii pitched for Mitsubishi in the industrial leagues after high school, missing two years due to shoulder problems. He was picked in the 6th round of the 2004 draft by the Chunichi Dragons. In his NPB debut on April 13, 2005, he served up a gopher ball to Tomonori Maeda on his first pitch. He won his first game just four days later. For the 2005 season, his numbers were unimpressive (3-1, 5.85, .348 opponent average, 2.07 WHIP). He improved to 2-1, 4.01 in 2006. He made one appearance in the 2006 Japan Series, allowing two hits, a walk and two runs while only retiring one batter. During 2007, he was 2-2 with a 2.95 ERA. In the 2007 Japan Series, he retired his only batter, relieving Kenichi Nakata in Game 2 and being relieved in turn by Rafael Cruz. Ishii thus played a very minor role in helping the Dragons win their first Japan Series since 1954.

Chunichi then dealt Ishii to his hometown Yokohama BayStars in exchange for outfielder Masaaki Koike, a former 20-homer man who had fallen into a backup role. Yuya was 2-0 with a 2.38 ERA, .177 opponent average and 0.97 WHIP as a solid reliever for Yokohama in 2008. He fell to 0-6 with 6 saves and a 4.26 ERA in 2009.

The BayStars traded Ishii to the Nippon Ham Fighters for fellow reliever Shintaro Ejiri. In 2010, he missed time due to a back injury and appeared in only 10 2/3 innings over 13 games (8 H, 2 BB, 8 K, 3 R). In 2011, he was 3-1 with a 1.31 ERA and .225 opponent average as a LOOGY (34 1/3 IP over 39 G).